Near-Infrared Fiber-Coupled Off-Axis Cavity-Enhanced Thermoelastic Spectroscopic Sensor System for In Situ Multipoint Ammonia Leak Monitoring

A near-infrared ammonia (NH3) sensor system based on a novel fiber-coupled off-axis cavity-enhanced thermoelastic spectroscopy (OA-CETES) was demonstrated for the first time. A quartz tuning fork (QTF) with a high Q-factor of ~12 000 was served as a detector and placed after a 6-cm-long off-axis integrated cavity with a finesse of ~482. The proposed fiber-coupled scheme was adopted exploiting a 100-m-long single-mode and multimode optical fiber for long-distance and multipoint sensing. Dual-laser modulation schemes of wavelength scanning and locking were comparably used for performance improvement. By targeting the NH3 line at 6612.71 cm−1, the developed OA-CETES sensor achieved a minimum detection limit (MDL) of 8.5 parts per million (ppm) for a 100-ms integration time and a normalized noise equivalent absorption (NNEA) coefficient of $1.7\times 10^{-9}$ cm−1 WHz−1/2. A fast-response NH3 leak monitoring with single-channel and multichannel cycling schemes was separately performed to verify the reliability of the proposed sensor system for field-sensing applications. The realization of the proposed OA-CETES with a small-size QTF (1–2 cm long) and fiber-coupled method allows a class of optical sensors for long-distance (up to kilometers) and multiplexed multipoint measurement and distributed sensing.